1. Technical Field
The present invention relates to an inkjet image forming apparatus.
2. Background Arts
In a line-type inkjet image forming apparatus, plural ink heads are aligned along a direction perpendicular to a feed direction of print sheets, and printing is done by injecting ink droplets from the ink heads onto a print sheet that is being fed. The ink heads in such an inkjet image forming apparatus have different injection properties, so that a density difference may occur with respect to neighboring ink heads due to the different injection properties. If a density difference occurs with respect to neighboring ink heads, the density difference may be recognizable easily. Therefore, it is required to reduce such a density difference.
In a prior-art inkjet image forming apparatus, there is proposed is a method adjusting density by (1) printing a print sheet with a print pattern by which a density difference is easily recognizable, or with an intermediary number of injected ink droplets by which a density difference is easily recognizable, (2) retrieving density of the print sheet by a scanner, and then (3) increasing or decreasing injection amounts of ink by relatively changing drive voltages (injection voltages) of neighboring ink heads (see Japanese Unexamined Patent Application Publication No. 2006-137040 [Patent Document 1], and Japanese Unexamined Patent Application Publication No. 2011-218657 [Patent Document 2]). Specifically, as shown in FIG. 7, density for dot (pixel) data 101 is increased by increasing a dot gain of a printed dot 103 by raising a drive voltage to make interspaces in a dot matrix 105 narrower. On the other hand, density for dot (pixel) data 101 is decreased by decreasing a dot gain of a printed dot 107 by lowering a drive voltage to make interspaces in a dot matrix 109 wider.
In addition, other than the density adjustment by changing drive voltages, there is a way for adjusting density through an image processing such as halftone processing in which the number of ink droplets per unit area is increased or decreased (see, Japanese Unexamined Patent Application Publication No. 2011-83992 [Patent Document 3]). Specifically, as shown in FIG. 8, when density of 4×4 dot (pixel) matrix data 201 is increased by a halftone processing, a dot matrix 203 is printed by increasing the number of ink droplets for dots 203a to 203d. On the other hand, when density of 4×4 dot (pixel) matrix data 201 is decreased by a halftone processing, a dot matrix 205 is printed by decreasing the number of ink droplets for dots 205a to 205d. 